The river Schelde is 355 km long, originating on the plateau of Saint-Quentin in France and ending in the North Sea in the Netherlands near Vlissingen. The Schelde estuary is approximately 160 km long and has a complete salinity gradient from polyhaline to a tidal freshwater zone, including extensive freshwater, brackish and saline tidal mudflats and marshes. It is a well-mixed estuary characterized by strong currents, high turbidity and large tidal amplitude (up to 6 m). The river has gone through dramatic and abrupt changes since the beginning of the millennium: especially since 2007 when the purification of the Zenne tributary - which receives the wastewater of Brussels - set in motion a chain of events:

• a reduced input of organic pollution into the Zeeschelde
• an anoxic zone in the river was resolved allowing marine fish, shrimp and mysid shrimp to reach the freshwater zone. These organisms often feed on Oligochaetes.
• due to the reduced food supply and the increasing predation of marine migratory organisms, the numbers of aquatic Oligochaeta which reached up to 5.000.000 worms m^-2 plummeted
• high numbers of wintering waterfowl declined only months after the purification
• surprisingly, so far the overall species richness of aquatic Oligochaeta did not change much: in general, the species richness stayed the highest in the more upstream parts of the estuary and subtidal samples seem to have some extra species, especially in upstream river parts.